1. Field of the Invention
The invention relates to a treatment to convert ferrous sulphate heptahydrate, which is a by-product of the production of titanium dioxide by the sulphate process, into a non-polluting particulate product.
2. Description of the Prior Art
As is known, the production of titanium dioxide by the sulphate process essentially involves the following operations:
(a) attack of the ilmenitic material with sulphuric acid and formation of a solution of iron and titanium sulphates;
(b) reduction of the iron to the ferrous state and its partial precipitation in the form of ferrous sulphate heptahydrate;
(c) hydrolysis of the resulting titaniferous solution and precipitation of the hydrated oxide of titanium;
(d) treatment of this hydrated oxide until titanium dioxide is obtained in the form of anatase or rutile;
(e) possible recovery of the sulphuric acid from the residual solution after hydrolysis.
The ferrous sulphate heptahydrate which is separated in stage (b) is in fact a mixture of several components of which a typical composition by weight is: FeSO.sub.4.7H.sub.2 O=88%, MgSO.sub.4.7H.sub.2 O=6%, TiOSO.sub.4 =0.3%, Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O=0.3%, MnSO.sub.4.5H.sub.2 O=0.2%, (VO).sub.2 SO.sub.4 =0.005%, free sulphuric acid 1.0%, water 4.0%, as well as several ppm of sulphates of zirconium, hafnium and rear earths. A product similar to that already described separates in stage (e) of the sulphate process, in that in this stage the residual solution from hydrolysis is treated, for example by cooling, to induce the separation of the residual iron sulphate.
Hence, in the description that follows, by ferrous sulphate heptahydrate, or more simply ferrous sulphate, will be meant those by-products, having a composition equal to or similar to that given above, which result from stage (b) and/or stage (e) in the production of titanium dioxide by the sulphate process.
The polluting characteristics of ferrous sulphate are such that they do not allow its direct disposal, neither into the sea nor into pits or the like. In fact in the first case the reducing properties of the ferrous salt, the phenomena of hydrolysis of the salt and other phenomena, induce serious damage to marine flora and fauna. In the second case considered, the leaching effect of water, especially rain water, results in pollution of the underlying water-bearing layers.
Moreover, in the process for the production of titanium dioxide about five parts by weight of ferrous sulphate are formed for every part by weight of titanium dioxide and account must be taken of the fact that modern installations have an annual output, with reference to titanium dioxide, of the order of tens of thousands of tons.
As a result the problem of disposing of very large quantities of a highly polluting product exists.
The conversion of the ferrous sulphate into products which are insoluble in water and non-hydrolysable by the latter, even if this appears to be an obvious solution at least at first sight, has not until now brought any satisfactory results in view of the numerous difficulties which are encountered in a treatment of this type. It is, in fact, difficult to ensure:
complete conversion or substantially complete conversion of the ferrous sulphate;
in industrially-acceptable times
the maintenance in the reaction mass of those free-flowing characteristics which are indispensable to a correct conduction of the reaction.
With regard to the first condition it is evident that incomplete neutralization does not allow the elimination of the pollution characteristics of the ferrous sulphate. The free-flowing of the reaction mass constitutes, on the other hand, a condition necessary for the carrying out of the reaction in the solid phase, it being taken into account that the use of an aqueous carrier would increase the mass in play to an intolerable extent and would bring with it additional costs for the separation of the carrier at the end of the reaction. Finally, short reaction periods also constitute a determining element of whether or not the process is economical.